Olefin dimerization

ABSTRACT

Olefins are dimerized in a loop reactor with flashing of the reactor effluent in a flashing zone within the loop. A subsequent flash in a second flashing zone can also be used for the removal of product as flash vapor. In another embodiment, separated product dimers or product heavies are used as absorbents for unreacted ethylene. The use of a vapor-liquid contacting device incorporated in the loop reactor is especially helpful in the dimerization of an olefin when the olefin is available in low-concentration gas streams. A thermosiphon loop reactor can also be used for low-concentration olefin streams to minimize the power requirements as the energy of the feed gas is used to induce the reactor circulation.

BACKGROUND OF THE INVENTION

This invention relates to the dimerization of olefins in a loop reactor.In another aspect, this invention relates to the dimerization of olefinsin a loop reactor that has a flash zone located within the loop therebyallowing the product to be removed as a vapor from the flash. In anotheraspect, this invention relates to the dimerization of olefins whereintwo flashing zones are used to remove the product as a vapor. In stillanother aspect, this invention relates to the concentration of anonvolatile homogeneous catalyst for subsequent recycling of catalyst tothe reactor loop by flashing off product. In yet another aspect, thisinvention relates to a dimerization process of olefins wherein productheavies are used as an absorbent in an absorber. in still anotheraspect, the invention is concerned with the use of product heavies toabsorb unreacted olefin and recycle the olefin to the loop reactor in adimerization process. In still another aspect, this invention relates toa dimerization process in which dimer product is utilized as anabsorbent for unreacted ethylene. The invention also relates to adimerization process for olefins in a loop reactor in which avapor-liquid contact device is contained within the loop and dimerproduct is used as an absorbent for unreacted ethylene within thedevice. The invention is also concerned with the use of the vapor-liquidcontacting device contained within the loop of a loop reactor forlow-concentration olefin feeds. In still another aspect, this inventionrelates to a dimerization process for olefins in a thermosiphon loopreactor using the energy of the feed gas to induce reactor circulation.This invention also relates to the use of a thermosiphon loop reactor inorder to dimerize olefins in a low-concentration feed.

Another aspect of this invention relates to the apparatus used todimerize olefins. in one aspect, the invention relates to olefindimerization apparatus comprising a flashing means in the continuousloop of a loop reactor. In another aspect, this invention relates toolefin dimerization apparatus comprising two flashing means. In anotheraspect, this invention relates to olefin dimerization apparatuscomprising a vapor-liquid contactor in the continuous loop of a loopreactor. In still another aspect, this invention relates to olefindimerization apparatus comprising a thermosiphon loop reactor.

The dimerization of olefins is a well-known process in the art, e.g.,U.S. Pat. No. 3,631,121 and U.S. Pat. No. 3,485,881. Olefin dimerizationprocesses are applicable to olefins in general, however, dimerization isan especially attractive method for producing butylenes from ethylenefor subsequent use in alkylation, dehydrogenation to butadiene and otherchemical processes. Problems arise in dimerization processes, however,in that the process suffers from low selectivity to the dimer with muchof the feed being converted to trimers and product heavies. It is knownthat selectivity can be improved by using shorter reactor residencetime, but the disadvantages of this approach are low ethylene conversionand low catalyst productivity.

Other problems arise when the feed stream is low in olefinconcentration, e.g. only a small amount of ethylene with the remainderof the feed stream being gases such as hydrogen, methane, ethane, etc.It would be desirable to remove the ethylene from the gas before the gasends up as a fuel gas stream. The amount of olefin, e.g., ethylene, isso small, however, that it would be economically undesirable to useconventional equipment with the expensive power requirements ofcirculating the reaction medium through the loop reactor as well ascostly recompression and the expense of a low temperature olefinrecovery column.

Accordingly, it is an object of this invention to provide an improvedand more economical process for olefin dimerization.

Another object of this invention is to provide an olefin dimerizationprocess with improved olefin selectivity and conversion as well ascatalyst productivity.

Another object of this invention is to provide an olefin dimerizationprocess which recycles the olefin reactant yet avoids costlyrecompression and an expensive low temperature recovery column.

Another object is to provide a method for dimerizing an olefin from alow concentration stream to thereby obviate the need for olefinpurification facilities.

Still another object of the invention is to minimize the powerrequirements of an olefin dimerization process using a loop reactor.

Other aspects, objects and advantages are apparent from a study of thisdisclosure, the drawings and the appended claims.

SUMMARY OF THE INVENTION

A process system has been devised which overcomes the deficiencies ofprior art olefin dimerization processes such as low olefin conversionand low catalyst productivity as well as low selectivity of the olefinto the dimer. The present invention is also concerned with variations inthe process system to enable one to minimize the power requirements ofan olefin dimerization reaction. Embodiments of the invention havespecial application for the dimerization of low-olefin-concentrationstreams. This is especially true for a gas stream containing a lowconcentration, 20% by volume or less, of an olefin such as ethylenewhere an ethylene plant as such is not available. It is desirable toseparate the olefin but not at the expense of building or usingexpensive olefin purification facilities. The dimerization processes ofthe invention can, therefore, be used to dimerize the olefin for easierseparation without the expense of complex olefin purificationfacilities. The process is suitable for use with any dimerizationcatalyst.

In a first embodiment, most of the major product is removed as a vaporby flashing the reactor effluent in a flashing zone. The flashing zoneis within the loop of the loop reactor and can, e.g., be the shell sideof a heat exchanger. By removing most of the major product as a vaporfrom the flash, the catalyst can be retained in the reactor for a longerperiod of time, which thereby improves catalyst productivity.

In another embodiment, the reactor effluent is flashed in two stagesthereby vaporizing most of the product and concentrating a catalyst in asmall stream of heavies for recycle to the reactor. Unconverted olefinis recovered in an absorber utilizing a heavies product stream, which isa product of the process, as the absorbent. The absorbed olefin canthereby be recycled to the reactor in the liquid phase, avoiding costlyrecompression and the expense of a low temperature olefin recoverycolumn.

The invention also contemplates the use of a liquid gas absorbercontactor within the loop reactor in order to concentrate the olefin andthereby obviate the need for olefin purification facilities. Productdimer can be used as the absorbent and can be recycled from the productstripper for such use. This embodiment is especially useful for lowolefin concentration streams, e.g., ethylene in a fuel gas stream.

In another embodiment, a thermosiphon type loop reactor is utilized tominimize the power requirements of the loop reactor in that the energyof the feed gas is used to induce reactor circulation. This embodimentis especially applicable for low-concentration olefin streams, e.g.,about 20% by volume or less, wherein the dimer is more easily removedfrom the gas stream than the original olefin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the apparatus used in the olefin dimerizationprocess wherein the product is removed from the loop reactor in aflashing zone.

FIG. 2 is a schematic showing an olefin dimerization process wherein thereactor effluent is flashed in two stages. The drawing also shows therecycle of unreacted olefin absorbed in product heavies.

FIG. 3 shows the embodiment of the invention wherein a liquid-gascontactor is contained within the loop of the loop reactor. Productdimer is used as the absorbent for the unreacted ethylene within thecontactor.

FIG. 4 is a schematic of the apparatus used in the olefin dimerizationprocess wherein a thermosiphon-type loop reactor is utilized.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to the dimerization of olefins. The dimerizationcan take place in a loop reactor, a type of reactor which is well knownin the art. The process and apparatus of the invention are suitable foruse with any appropriate dimerization catalyst. The catalysts preferablyemployed, however, are those of any hydrocarbon-soluble nickel compound,alkyl aluminum halide, or mixture thereof, e.g., tri-n-butylphosphinenickel dichloride mixed with ethyl aluminum dichloride orbis(tri-n-butyl-phosphine)dichloronickel.

When the catalyst employed is a mixture of a hydrocarbon-soluble nickelcompound and an alkyl aluminum halide, it is preferred that lowaluminum/nickel molar ratios, e.g., molar ratios in the range of about2.1 to about 7, are used in the catalyst components in order to minimizefouling. Further, with the use of low aluminum/nickel ratio catalyststhe deposits that do form in the reactor and heat exchangers can bereadily removed by washing with a 10 weight percent acetic acidsolution.

Another factor in minimizing fouling of the apparatus is the materialflow fluid velocity in the continuous, closed reaction zone. Themaintenance of high fluid velocities, e.g., 6-20 feet/second, in thecirculating reaction loop helps to minimize the formation of deposits.It is preferred to maintain a velocity of about 12 feet/second or higherfor greatest success at minimizing fouling.

Suitable process conditions of temperature and pressure can vary greatlyfor this invention and can be easily determined by one skilled in theart.

The process is applicable to the dimerization of any suitable olefin,with the invention especially applicable to the selective dimerizationof C₂ to C₁₀ olefins. Ethylene is the preferred olefin reactant,however, as ethylene dimerization is an attractive method for producingbutylenes from ethylene for subsequent use in alkylation,dehydrogenation to butadiene and other chemical processes.

A better understanding of the invention will be obtained upon referenceto the drawings. Although the drawings are discussed with reference toethylene as the olefin reactant to be dimerized, it is emphasized thatthis should not limit the invention in any way. The invention isapplicable to any olefin that can be dimerized but the discussion islimited to ethylene as that is the preferred olefin reactant.

FIG. 1 depicts an embodiment of the invention wherein ethylene isdimerized in a loop reactor and reactor effluent is flashed in aflashing zone located within the loop, thereby removing part of theexcess thermal heat of the reaction and allowing part of the productdimer to be removed as flash vapor. The olefin reactant, e.g., ethylene,is introduced into a continuous closed path reaction zone 1, e.g., aloop reactor, via conduit 2. Catalysts selective to dimerization areadded via 3 and 4, or, if only one catalyst were to be used, thecatalyst can be added via either 3 or 4. The olefin reactant, ethylene,is thereby reacted in the presence of said catalyst selective todimerization under such conditions as to result in the dimerization ofthe olefin. The reactor effluent is then flashed in a flashing zone 5located within the continuous closed path reaction zone 1. Product dimeris removed via 6 from the flashing zone as flashed vapor. As an option,one can place a disengaging tank in line 6 thereby assuring that anyentrained liquid would be separated from product vapor and returned tothe flashing zone. The flashed vapor is passed through variousseparation units to yield a purified dimer product.

A portion of the flashed liquid from flashing zone 5 is recycled to thereaction zone via conduits 7 and 8. The remainder of the flashed liquidis passed to a recovery system, e.g. fractionator 10, for recovery ofproduct dimer via conduits 7 and 9.

The material flow or process velocity of the continuous closed pathrecovery zone can vary, however, it is preferred to maintain thematerial flow at about 12 feet per second or higher as opposed to themore conventional flow of 6-8 feet per second. Maintaining the materialvelocity at such a rate can increase or extend the interval betweencleaning the system from about 400 to about 4,000 hours, or ten times aslong.

The flashed vapor from flashing zone 5 can be passed via 6 to anabsorption zone 11 in order to recover the product dimer, as well as anyproduct heavies, i.e., trimers, etc., from unconverted ethylene or othergases contained in the flashed vapor. The unconverted ethylene isremoved via 12 and the product dimer and heavies are passed via 13 tofractionator 10. At fractionator 10, the product dimer, butene, isseparated from the product heavies and recovered via line 14. Thebottoms of the fractionator are composed of product heavies which areremoved via 15 and passed to disposal or storage via 16 or recycled toabsorber 11 via conduit 17 and cooler 22. The heavies are introducedinto absorber 11 near the top 18 and act as an absorbent for the productdimer.

The flashing zone 5 can be a heat exchanger located within the loopreactor with the reactor effluent flashing in the shell side 19 of saidheat exchanger. The heat exchanger, therefore, serves the dual purposeof cooling the solvent and reaction effluent, as well as removing mostof the major product, butenes, as a vapor by flashing. By removing mostof the butenes as a vapor from a flash, the catalyst can be retained inthe reactor for a longer period of time and thereby improve catalystproductivity. The removal by flashing also eliminates a catalyst killingand removal step which would normally accompany a separation wherein theentire reactor effluent was passed to the separation facilities. Asmentioned above, one can optionally place a disengaging tank in line 6.The disengaging tank assures that any entrained liquid would beseparated from product vapor and thereby be returned to the heatexchanger.

The vaporization of most of the product by flashing concentrates thecatalyst in a small stream of heavies for recycle to the reactor viaconduits 7 and 8. A buildup of heavies in the system is controlled bydirecting a portion of the recycled flash liquid to the productfractionator via conduits 7 and 9. As indicated in FIG. 1, the flow ofthis stream to the product fractionator can be controlled by a densitycontroller 20, or any other suitable analyzer means, on therecirculating reactor loop stream. It is also desirable to kill thecatalyst in this stream passing to product fractionation in order toprevent the production of undesirable heavies in that column. This isdone by the introduction of a small amount of water or steam 21 into thefeed to the product fractionator.

By controlling process variables, i.e., reactor temperature, residencetime, etc., ethylene conversion can be maintained at 99 percent,eliminating the need for ethylene recycle.

FIG. 2 depicts an embodiment of the invention wherein the reactoreffluent is flashed in two stages. Gaseous ethylene or other selectedolefin feed from line 52 enters the continuous closed path reaction zone51 at 53. In order to initiate the dimerization reaction, suitableamounts of one, two, or more suitable catalysts are injected into theloop reactor system via 54. Examples of suitable catalysts are a mixtureof ethyl aluminum dichloride and tri-n-butylphosphine nickel dichlorideor just bis(tri-n-butylphosphine)dichloronickel. The conditions in theloop reactor are such as to promote the dimerization of the olefin,ethylene, in the presence of the catalyst. Circulation through thecontinuous closed path reaction zone is maintained by means of pump 55.

The reactor effluent is then flashed in flashing zone 56 which islocated within the continuous closed path reaction zone. Overhead fromthis first flash zone passes to a deethanizer 57 via conduit 58. Theresultant or residue liquid from the first flashing zone is passed viaconduit means 59 through heat exchanger 60 to a second flashing zone 61.This second stage flash is at a lower pressure than the first stage andthereby removes product dimers, e.g., C₄ hydrocarbons, as well asproduct heavies, e.g., C₈, etc. The overhead from the second flash isremoved via 62 and passed to fractionator 63 via conduit means 64.Fractionator 63 divides or separates the product stream into its variouscomponents, including the product dimer butenes which are removedoverhead at 65. Bottoms of the fractionator 66, which comprises octenesand other product heavies are passed to disposal or storage via 67 witha portion recycled via 68 and heat exchanger 69 to the top of thedeethanizer 57. The heavies stream is used as an absorbent for theunconverted ethylene. An ethylene-rich side stream is taken from theabsorber at 70 and pumped by means of pump 71 back to the reactor viaconduit 72. The use of the absorption section on a low-pressure columnto recover the unreacted ethylene using a product heavies fraction asthe solvent permits the operation of the fractionator train at a lowpressure on flashed reactor effluent thereby allowing the absorbedethylene to be recycled to the reactor in a liquid phase and avoidingcostly recompression and the expense of a low-temperature ethylenerecovery column.

Overhead inert gases from the deethanizer are removed via 73 and passedon for use as fuel gas. Bottoms from the deethanizer are passed via 74to fractionator 63.

The bottoms of the second flash zone 61, which contain primarily spentcatalyst, are removed via 75 and pumped back to the reactor. The spentcatalyst, which has in the prior art been precipitated by hydrolysiswith minute amounts of water entering with the feed, is removed with afilter 76 to avoid fouling of the heat exchange surface in the reactorloop.

The buildup of heavies in the system is controlled by directing aportion of the recycled second stage flash liquid to the ethyleneabsorber via 77, or, optionally, to the product fractionator. Asindicated in FIG. 1 at 20, the flow of this stream can be controlled bya density controller or other analyzer means on the recirculatingreactor loop stream.

It is desirable to kill the catalyst in the stream passing to theproduct fractionator 63 in order to prevent the production ofundesirable heavies in that column. This can be done by the introductionof a small amount of water or stream into the feed to the productfractionator as depicted at 21 in FIG. 1. However, when the two-stageflash system is used as shown in FIG. 2 steam can be used in an ejecter78 on the second stage flash vapor in order to also maintain the desiredlow pressure in the second stage flash vessel. Any suitable compressionmeans can be substituted for the ejector.

The first flash zone 56 can be a heat exchanger located within thecontinuous closed path reaction zone as was discussed previously inrespect to the embodiment shown in FIG. 1. When a heat exchanger isused, the reactor effluent is flashed into the shell side 79 of saidheat exchanger.

The flashing of the reactor effluent in two stages allows recovery ofnearly all the product as flashed vapor and thereby avoids the loss ofactive catalysts from the system. By reducing the catalyst loss from thereactor with reaction products, the catalyst usage would be decreasedand productivity pushed even higher.

Referring to FIG. 3, an embodiment of the invention is depicted whereina vapor liquid contacting tower is incorporated within the loop reactor.The vapor liquid contacting tower, e.g., a packed tower, is shown at101. The vapor-liquid contacting tower shown in FIG. 3 comprises a lowersection 102 which is used as a reacting zone with bottoms recycled tothat zone and an upper section 103 which serves as an absorption zone.

Olefin feed, for example, an ethylene stream, enters the lower sectionof the tower 104. Catalyst used to initiate the dimerization reactioncan be introduced into the system via 105 and 106 near the bottom of theabsorber. Solvent or absorbent is introduced near the top of the towerat 120. The absorbent recovers unreacted ethylene with dimer product,product heavies or any suitable absorbent or solvent for ethylene thatis known in the art as being an appropriate absorbent. The solvent hasbeen recycled from product stripper 115 via conduit 117. The loopreactor is maintained at a temperature and pressure sufficient toliquefy butenes. The reaction stream in the continuous closed pathreaction zone is continuously circulated by a pump 107. The reactionstream is also cooled by indirect heat exchanger 108 and returned to anintermediate point of the absorber at 109. Unreacted or unabsorbed gasessuch as hydrogen and methane are removed as absorber overhead at 110 andpassed to storage or for further use as fuel gas.

A product side stream is continuously removed from the loop at 111 andpassed via conduit means 112 to a catalyst removal zone 113. The productstream is then passed via 114 to product stripper 115 from which lightsare vented overhead at 116, and product dimer is recovered at 121. Theproduct dimer is cooled at 118 and passed to storage.

In another embodiment of the invention, a portion of the product dimeris passed via 117, cooled at 119 and introduced into the top of theabsorber at 120. The recycled product dimer, therefore, functions as anabsorbent to recover unreacted ethylene. In general, the solvent isrecycled via 117, 119 and introduced at 120, whether it be dimerproduct, product heavies, or some other appropriate ethylene solvent.The absorbent recovers unreacted ethylene and keeps the ethylene frombeing lost overhead. The absorbent is removed in the product stripper at117 and is continuously recycled to the top of the absorber tower. Ifthe product dimer is used as absorbent, the dimerization of ethylenecontinually supplies additional dimer solvent to replace that beingremoved as product.

This embodiment has special application to low-concentration olefinstreams, namely, gas streams with an olefin concentration of up to about20 percent by volume. An example of such a stream is a dry gas ventstream from an ethylene cracking unit which can contain up to about 20percent by volume ethylene, e.g., five percent of the product ethylene.The vent stream comprises such gases as hydrogen, methane, ethane, etc.,and is usually used as field gas stream whereby the ethylene, unlessseparated, is lost. The use of ethylene purification facilities,however, would be very costly due to the low concentration of ethylenein the feed stream.

The use of the present invention, however, obviates the need forethylene purification facilities other than for the removal of catalystpoisons such as water and sulfur compounds. The ethylene in the feed gasis dimerized to butenes which are much more easily recovered andseparated from the gases than ethylene. As well, the recycled dimerkeeps ethylene from being lost overhead to the field gas stream. Thedimerization of ethylene to butene in order to recover it from fuel gasstreams using a loop reactor containing an absorber or contacting towerwithin the loop is a much more efficient and economical process for therecovery of ethylene from a feed stream when the ethylene is present ina low concentration. FIG. 4 depicts another embodiment of the inventionin which a thermosiphon-type loop reactor, such as the one disclosed inU.S. Pat. No. 3,213,157, is utilized to dimerize an olefin feed. Athermosiphon loop reactor is especially applicable tolow-olefin-concentration feeds as the thermosiphon reactor minimizes thepower requirements for the dimerization process in that it utilizes theenergy of the feed gas to induce reactor circulation. A pump, therefore,is not needed to circulate the low concentration feed continuouslythrough the loop reactor.

Referring now to FIG. 4, an olefin feed, which can be alow-concentration ethylene feed containing only about 20% by volume orless, e.g., approximately 5% ethylene, is introduced into loop reactor151 via conduit 152. The energy of the feed gas is used to circulate thereactor liquid through the reactor. No pump is required in thiscontinuous closed path reaction zone as previously depicted in FIGS.1-3.

Catalyst, selective to dimerization, is introduced into the loop at 153via conduits 154 and 155. The circulating liquid catalyst solvent shouldbe a high boiling point material so that its vapor pressure would bevery low at 100° F. (37.8° C.) in order to prevent excessive solventloss to the gas stream. Additional make-up solvent can be added at 168.

The circulating reactor effluent passes through a separation zone 156 inwhich vapor is separated from liquid. Due to its vapor pressure and thelarge amount of "inert" gas, the product dimer butenes will remain withthe gas stream until they are either chilled out or scrubbed out of thefuel gas stream. The product butenes and fuel gas stream are therebyremoved overhead 157 and passed via conduit 158 to chiller 159. Theoverhead is then passed via 160 to a phase separation zone 161 fromwhich fuel gas is recovered as overhead 162 and butene product isrecovered as bottoms 163 which is then passed to recovery zone 164.Purified butene product is then recovered as bottoms 165 from saidrecovery zone.

A portion of the bottoms from separation zone 156 is removed at 166 inorder to reprocess any spent catalyst. The remainder of the catalyst inbottoms liquid is recycled via 167 to the reactor.

In another embodiment of the invention (not shown in the drawing), anexternal absorbent, such as dimethylsulfoxide, is used to recoverethylene and heavies from the reactor effluent. The reactor effluent iscontacted with the absorbent to remove the hydrocarbons and yield fuelgas as overhead. The hydrocarbons are then separated from the absorbentand charged to fractionation from which product dimer, e.g., butenes, isrecovered and olefin, e.g., ethylene, is recycled to the dimerizationreactor.

The process shown in FIG. 4 has the advantages that since the productbutenes remain with the gas stream 158, no special catalyst removal ortreatment of the product stream is required. Also, the use of thethermosiphon reactor minimizes energy requirements for circulating thematerial through the reactor since the energy from the feed gas is usedto induce this circulation. The use of this gas lift-type reactor isespecially economical energy-wise when using a low concentrationethylene feed due to the high concentration of inert gases.

The following examples demonstrate the invention with respect to theembodiments shown in FIGS. 1, 2, 3, and 4. The following embodiments arenot intended to limit the invention in any way and are only given forillustration.

EXAMPLE I

The invention was demonstrated in a pilot plant loop reactor made up of40 feet of one-inch OD stainless steel tubing with a total volume ofabout 2.6 gallons. Feed was high purity ethylene (99.9 wt. %) charged at23.5 lbs/hr. Catalyst was a complex of nickel (II) chloride andtri-n-butylphosphine, namely, bis(tri-n-butylphosphine)dichloronickel.Reactor conditions were 92° F. (33° C.), 174 psia, and 33 minutesresidence time.

Using the single flash reactor configuration of FIG. 1, ethyleneconversion was 93.4 percent and catalyst productivity was 45,468 lbs.product per lb. nickel when flashed product was not recycled to thereactor loop. With recycle of flashed product as shown in FIG. 1, it hasbeen calculated that catalyst productivity would increase to 389,600pounds of product per pound of nickel. Compositions for the reactoreffluent and the calculated flash at 90° F. (32° C.) and 30 psia are:

    ______________________________________                                        Stream Compositions, Mole Fraction                                                     Reactor   Flash      Flash                                                    Effluent  Vapor      Liquid                                          ______________________________________                                        Stream No.             (6)        (7)                                         Ethylene   0.111       0.142      0.007                                       Butene-1   0.040       0.045      0.026                                       t-Butene-2 0.529       0.563      0.417                                       c-Butene-2 0.228       0.239      0.191                                       Heavies    0.092       0.011      0.359                                       Total      1.000       1.000      1.000                                       ______________________________________                                    

EXAMPLE II

Calculations have been made to illustrate operation in the preferredmode of operation illustrated in FIG. 2. Reactor conditions are the sameas for Example I, as are conditions for the first flash. Liquid from thefirst flash is further flashed at 90° F. (32° C.) and 5 psia to yield aheavies concentrate containing the catalyst which is recycled to thereactor.

    ______________________________________                                        Stream Compositions, Mole Fraction                                                             First    First  Second Second                                                 Flash    Flash  Flash  Flash                                         Reactor  Vapor    Liquid Vapor  Liquid                                Stream No.                                                                            Effluent (58)     (59)   (62)   (75)                                  ______________________________________                                        Ethylene                                                                              0.1000   0.1531   0.0070 0.0101 0.0001                                Butene-1                                                                              0.0386   0.0453   0.0267 0.0370 0.0040                                t-Butene-2                                                                            0.5042   0.5566   0.4123 0.5668 0.0746                                c-Butene-2                                                                            0.2169   0.2340   0.1871 0.2540 0.0410                                Heavies 0.1403   0.0110   0.3669 0.1321 0.8803                                Totals  1.0000   1.0000   1.0000 1.0000 1.0000                                ______________________________________                                    

With this mode of operation, calculated catalyst productivity is about9,000,000 pounds of product per pound of nickel.

EXAMPLE III

Calculations have been made to illustrate operation of the embodimentshown in FIG. 3. The illustrative process conditions are: a solvent tofeed weight ratio of about 0.5, a reactor temperature of about 100° F.(38° C.), and reactor pressure of about 100 psia. The calculatedmaterial balance for the process shown in FIG. 3 is as follows:

    __________________________________________________________________________               104      105 106 110                                               Stream No. lbs/day                                                                            wt. %                                                                             lbs/day                                                                           lbs/day                                                                           lbs/day                                                                            wt. %                                        __________________________________________________________________________    Hydrogen   944,602                                                                            76.8        941,010                                                                            80.3                                         Methane    224,143                                                                            18.2        213,955                                                                            18.3                                         Ethylene   61,512                                                                             5.0         2,691                                                                              0.2                                          Butenes                     14,609                                                                             1.2                                          Solvent                                                                       Nickel Catalyst         10.0                                                  Aluminum Catalyst   13.0                                                      Total      1,230,257                                                                          100.0                                                                             13.0                                                                              10.0                                                                              1,172,265                                                                          100.0                                        __________________________________________________________________________                 112 116       119  117                                                       lbs/day                                                                            lbs/day                                                                            wt. %                                                                              lbs/day                                                                            lbs/day                                       __________________________________________________________________________    Hydrogen    3,592                                                                              3,592                                                                              22.0                                                    Methane     10,188                                                                             10,188                                                                             62.3                                                    Ethylene    385  385  2.4                                                     Butenes     43,827                                                                             2,191                                                                              13.3 41,636                                             Solvent     615,129             615,129                                       Nickel Catalyst                                                                           10                                                                Aluminum Catalyst                                                                         13                                                                Total       673,144                                                                            16,356                                                                             100.0                                                                              41,636                                                                             615,129                                       __________________________________________________________________________

EXAMPLE IV

The process shown in FIG. 4 is illustrated in the following calculatedexample. The reactor conditions of temperature and pressure are the sameas in Example III.

    __________________________________________________________________________    MATERIAL BALANCE (LBS/DAY)                                                    Stream No.                                                                              152  154                                                                              155                                                                              158   162   163 169 165                                  __________________________________________________________________________    Hydrogen  944,602    944,602                                                                             944,580                                                                             22  22                                       Methane   224,143    224,143                                                                             223,704                                                                             439 439                                      Ethylene  61,512     3,076 2,699 377 377                                      Butenes              58,436                                                                              2.699 55,737                                                                            377 55,360                               Nickel Catalyst                                                                              1.0                                                            Aluminum Catalyst 1.3                                                         Total     1,230,257                                                                          1.0                                                                              1.3                                                                              1,230,257                                                                           1,173,682                                                                           56,575                                                                            1,215                                                                             55,360                               __________________________________________________________________________

Reasonable variations and modifications which will become apparent tothose skilled in the art can be made in the present invention withoutdeparting from the spirit and scope thereof.

I claim:
 1. In a process for dimerizing an olefin whichcomprises:introducing said olefin into a continuous closed path reactionzone, reacting said olefin in the presence of a catalyst selective todimerization and such conditions to result in the dimerization of theolefin, flashing the reaction mixture in a flashing zone to thereby forma flashed vapor and resultant liquid, recovering the product dimer fromthe flashed vapor of the flashing zone the improvement comprisingcarrying out said flashing in the shell side of a heat exchanger whichis located in said continuous closed path reaction zone thereby removingpart of the thermal heat of the dimerization.
 2. A process in accordancewith claim 1 further comprising the steps of:recycling a portion of theliquid flashed from the flashing zone to the reaction zone, and passingthe remainder of the liquid obtained from the flashing zone to arecovery system for product recovery.
 3. A process in accordance withclaim 1 wherein the material flow in said continuous closed pathreaction zone is maintained at a velocity in the range of about 6-20ft/sec.
 4. A process in accordance with claim 3 wherein said velocity isabout 12 ft/sec.
 5. A process in accordance with claim 1 wherein saidolefin is ethylene.
 6. A process in accordance with claim 1 wherein theproduct dimer is recovered from the flashed vapor in an absorption zonein which a portion of the product heavies is used as the absorbent.
 7. Aprocess in accordance with claim 1 wherein the liquid from the flashingzone is flashed in a second flashing zone, andthe flashed vapor fromsaid second flashing zone is passed to a fractionation zone for therecovery of product dimer.
 8. A process in accordance with claim 7wherein the flashed vapor from the first flashing zone is passed to anabsorption zone,the bottoms from the absorption zone is passed to thefractionator zone, product dimer is recovered as overhead from thefractionation zone with bottoms from the fractionation zone passed tosaid absorption zone as absorbent, and a side stream comprised of olefinand absorbent is withdrawn from said absorption zone and is recycled tothe reaction zone.
 9. A process in accordance with claim 7 wherein thebottoms from said second flashing zone is filtered and recycled to thereaction zone.
 10. In a process for dimerizing an olefin whichcomprises:introducing said olefin into a continuous closed path reactionzone, reacting said olefin in the presence of a catalyst selective todimerization under such conditions as to result in the dimerization ofthe olefin, the improvement comprising arranging the lower section of avapor liquid contacting zone in said continuous closed path, recoveringunreacted olefin with a solvent introduced into the upper section ofsaid liquid-vapor contacting zone, passing a portion of the bottom fromsaid vapor-liquid contacting zone to a recovery system for the recoveryof the dimer product and solvent with the remainder being recycled tothe vapor-liquid contacting zone and being introduced just above saidlower section of said vapor-liquid contacting zone, and recycling thesolvent to the vapor-liquid contacting zone as absorbent for unreactedolefin.
 11. A process in accordance with claim 10 wherein said olefin isintroduced as a low concentration stream of gases and said solvent isproduct dimer thereby recovering one portion of the dimer as product andrecycling a second portion to the vapor-liquid containing zone asabsorbent for unreacted olefin.
 12. A process in accordance with claim10 wherein said olefin is ethylene and said catalyst is a mixture of ahydrocarbon-soluble nickel compound and an alkyl aluminum halide.
 13. Aprocess in accordance with claim 12 wherein the aluminum/nickel molarratio in the catalyst components is in the range of about 2.1 to about7.
 14. A process in accordance with claim 10 wherein the material flowin the continuous closed path reaction zone is maintained at a velocityin the range of about 6-20 feet/sec.
 15. A process for dimerizing anolefin which comprises introducing a feed gas containing said olefininto a continuous closed path reaction zone including a verticallyextended reaction zone wherein the olefin is reacted in the presence ofa catalyst selective to dimerization under such conditions to result inthe dimerization of the olefin and the reaction mixture passes throughthe continuous closed path reaction zone solely by the energy impartedto said catalyst by the feed gas and density differential in thecontinuous closed path reaction zone.
 16. A process in accordance withclaim 15, wherein said olefin is in low concentration in said first gas.17. A process in accordance with claim 16 wherein said olefin isethylene.